Socket API

Chapter 4.4 tied the Transport Layer directly to real systems programming through the Berkeley Socket API: the 7 core primitives, the exact call ordering for TCP vs UDP servers and clients, why accept() returns a new connected socket descriptor while the welcome socket keeps listening, and which calls block waiting for network packets.

1. MCQ · 1 mark

   Which socket system call converts an unbound/bound socket into a passive listening socket and allocates kernel queues for incoming TCP connection requests?

   1. bind()
   2. listen()
   3. accept()
   4. connect()
2. MCQ · 1 mark

   When a TCP server calls accept() on a listening socket and a client connection is established, what is the return value of accept()?

   1. 0 on success, −1 on error.
   2. The port number of the client.
   3. A brand-new socket descriptor representing the established connection with that specific client.
   4. The same socket descriptor that was passed into accept().
3. NAT · 2 marks

   A concurrent TCP server binds to port 8080, calls listen(), and accepts connections from 25 concurrent clients (none of which have closed yet). Meanwhile, a UDP server on the same host binds to UDP port 8080 and exchanges messages with 25 clients. What is the difference between the number of socket descriptors open in the TCP server and the number of socket descriptors open in the UDP server?

   Write your answer before opening the key.

4. MCQ · 1 mark

   Why does a typical TCP client NOT call bind() before calling connect()?

   1. Because TCP clients do not have a source port number.
   2. Because the operating system kernel automatically assigns an available ephemeral source port when connect() is called on an unbound socket.
   3. Because bind() is only supported for UDP sockets.
   4. Because calling bind() on a client triggers a RST segment.
5. MSQ · 2 marks

   Which of the following socket system calls are BLOCKING by default (when the socket is in blocking mode)?

   1. accept()
   2. connect() on a TCP socket
   3. listen()
   4. recv() when the receive buffer is currently empty
6. MCQ · 1 mark

   What is the typical sequence of socket system calls for a connectionless UDP server?

   1. socket() → bind() → listen() → recvfrom() → sendto() → close()
   2. socket() → bind() → recvfrom() → sendto() → close()
   3. socket() → connect() → recv() → send() → close()
   4. socket() → accept() → recvfrom() → close()
7. NAT · 2 marks

   In a TCP connection between a client calling connect() and a server blocked on accept(), how many TCP segments are exchanged on the wire before connect() returns on the client?

   Write your answer before opening the key.

8. NAT · 2 marks

   A TCP Reno connection with MSS = 1.5 KB starts with cwnd = 1.5 KB (1 MSS) and ssthresh = 12 KB (8 MSS). At the end of RTT 4 (when cwnd = 12 KB), 3 duplicate ACKs arrive. What is cwnd (in KB) in RTT 5?

   Write your answer before opening the key.